chore: point every in-tree reference at ai-job-search-dev/ai-job-search (#776)

The repository is moving from the MadsLorentzen personal account to the
ai-job-search-dev organization. GitHub redirects the old slug, but the
upstream-watch guard compares github.repository against the literal slug
and would start running the fork-only triage job on upstream itself, and
the upstream-check tool and its tests pin the slug too. README, CONTRIBUTING,
SECURITY, SETUP, the CI workflow, the issue-template config and the
CHANGELOG links follow.

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

tests/_upstream.py

@@ -10,7 +10,7 @@ import os
 import subprocess
 from pathlib import Path
 
-UPSTREAM = "MadsLorentzen/ai-job-search"
+UPSTREAM = "ai-job-search-dev/ai-job-search"
 
 REPO = Path(__file__).resolve().parent.parent